1. Field of the Invention:
The present invention relates to an instrument for detecting the level of bilirubin in the skin. More particularly, the present invention relates to a noninvasive method of detecting bilirubin concentration in the skin of infants.
2. Discussion of the Background:
Bilirubin is a reddish to yellow pigment having the chemical formula C.sub.33 H.sub.36 N.sub.4 O.sub.6 and consisting of a broken porphyrin ring which is produced in the metabolic breakdown of hemoglobin and other porphyrin-containing proteins. In an adult it is removed from the blood by the liver and forms the major pigment of bile, through which it is excreted from the body. However, the liver of a newborn child, and especially of a premature infant, is not yet mature and cannot process bilirubin in this way.
The birth process often results in extensive bruising of the newborn: blood escapes into the tissue and must then be broken down metabolically. Bilirubin forms and, since the liver cannot remove it, accumulates in the bloodstream. If levels rise high enough, it begins to be deposited in other body tissues. Its first and most visible appearance is in the skin, where it creates a form of jaundice. At still higher levels, deposition begins in deeper tissues, including the brain, and can result in permanent brain damage from kernicterus.
To prevent damage, an infant's bilirubin levels must often be closely monitored for the first several days, and sometimes several weeks, of life. Once detected, an excess is easily removed from the body by exposure to ordinary blue light, which is strongly absorbed by bilirubin converting it to an activated form more easily processed by the immature liver. Skin-deposited bilirubin and blood-borne bilirubin when circulating through vessels near the skin, can be purged from the body through simple skin exposure to sunlight, or to lamps with a high bluelight output ("bili lights") for several hours a day over the course of a week or so.
Skin deposition of bilirubin is not uniform over the body, but begins, and remains heaviest, in the face and upper body. The result is a yellowish tinge which appears first in the face and then, as deposition continues, both darkens and extends downward to other skin areas. Initial upper-body bilirubin deposition may have evolved as a survival mechanism, since these areas would have been the most often (and most directly) exposed to sunlight. Color comparison may be a reasonable, noninvasive method of monitoring bilirubin.
Color comparison is complicated by the fact that other pigments, besides bilirubin, are present in the skin. Chief among these are melanin, the normal brown pigment of skin, and hemoglobin in the surface blood vessels. Melanin is less of a problem than it might first appear to be. Melanin formation is a slow and complex process, typically incomplete until several months or even years of age: newborns, of whatever race, have relatively little. The amount of melanin present, moreover, is normally uniformly distributed in the skin (with the exception of the so-called "mongolian spot" in some racial groups) and thus will not cause an interfering color difference between properly-chosen upper and lower body sites. For practical purposes, therefore, this source of interference may be disregarded. Hemoglobin presents a potentially more severe problem, since its pattern of distribution (in blood vessels) is inherently uneven.
Bilirubin monitoring, however, usually requires blood analysis, and this poses a problem. Because an infant's circulatory system is not yet fully developed, blood must be drawn using a procedure called a "heel stick" in which one or more punctures are made in the heel and blood is repeatedly squeezed out into a collecting tube. Obtaining the several milliliters needed for the analysis can take as much as twenty minutes. The procedure is traumatic even for an infant in good health, and for a very premature infant adds stress which combined with other factors could actually be life-threatening. Because of this, bilirubin testing is usually only begun when, in the judgment of a health professional, an infant already shows visible signs of jaundice.
A simpler method of bilirubin monitoring, requiring no blood-letting or other invasive procedures, would save time and money, spare infants the trauma of the "heel stick," and thus make practical the testing of all infants--not merely those already showing visible signs of jaundice--for elevated bilirubin. By eliminating the need for judgment calls by health professionals with varying levels of expertise, the method would permit more consistent measurements across time and would likely provide earlier detection of elevated bilirubin in some cases. Conceivably, by removing a major source of stress, such a method could also help save the lives of fragile premature infants.